The present invention relates to a brake or clutch component where at least a portion of the component is a ceramic-metal composite.
Generally, in an automotive disk brake, the brake rotor is attached to the car by an axle. As the car travels, the brake rotor rotates with the wheel of the car. The brake caliper assembly contains the brake piston and brake pads and is rigidly mounted to the car frame. Upon application of the brake pedal, hydraulic fluid drives the brake piston or pistons outward from the cylinders in the caliper assembly forcing the brake pads to engage the rotor causing the frictional braking force that stops the car from traveling.
Since the discovery of the toxic effects of asbestos, brakes for light duty vehicles, such as pickup trucks and cars, have been made from cast iron rotors or drums engaged by brake pads or shoes having semi-metallic or non-asbestos organic composite pads. These brakes have suffered from problems such as noise, shudder, vibration and short lifetimes of the pads, rotors and drums. Drums and rotors have also tended to warp due to greater heat generation as a result of the use of semi-metallic brake pads. A consequence of this has been excessive warranty costs to automobile manufacturers, which has been estimated to be about $85 per car in North America.
In addition, since the present pads or shoes wear out quickly, the hydraulic brake piston that forces the pad into contact with the rotor or drum has needed to have a long travel to compensate for the wearing out of the brake pad or shoe (that is, the pad has to be thicker to compensate for the faster wear rate). The long travel necessitates the use of a larger caliper assembly and larger piston, which adds weight to the brake. The excessive wear of today""s brake pads also cause aesthetic problems, such as brake pad dust on the wheels.
More recent alternative brakes, such as carbon/carbon composite brakes, have only been used, due to cost and design considerations, on the most exotic applications (for example, race cars and military aircraft).
Therefore, it would be desirable to provide a brake that is lighter, avoids the short lifetime, dusting and repair costs associated with today""s light duty vehicle brakes at a cost competitive with current metal brakes.
A first aspect of this invention is a braking component comprised of a metal substrate that has a friction material laminated onto at least a portion of one face of the metal substrate, wherein the friction material is a ceramic-metal composite comprised of a metal phase and a ceramic phase, the ceramic phase being present in an amount of at least 20 percent by volume of the composite. Another aspect of this invention is a brake having at least one braking component of the first aspect of this invention. A third aspect of this invention is a clutch having at least one braking component of the first aspect of this invention.
The braking component of the present invention may be used as a brake component, such as a brake rotor, brake drum, brake shoe and brake pad. The braking component may also be used as a clutch disk or flywheel. Examples of clutches include automotive drivetrain clutches, air conditioner clutches and compressor clutches in refrigerators. The braking components of this invention may be made with lighter metals having lower melting temperatures than metals currently used in brakes. The braking component consequently allows light duty vehicle brakes to be less massive. In addition, the use of the braking components generally provide reduced wear compared to current brakes, consequently, brakes made from these components can be smaller while providing the same lifetimes as current brakes. Similar enhancements result for clutches.
The Braking Component
The braking component may be any component that generates a braking or frictional force when contacted with an opposing component. In particular, the braking component is a component that contacts an opposing component moving relative to the braking component such that the relative motion of the two components is arrested. Examples of the braking component include brake pads, brake shoes, brake rotors, brake drums, clutch disks, flywheels and centrifugal chucks.
The braking component is comprised of a metal substrate having a friction material laminated to at least a portion of one face. In general, the metal substrate supports the friction material and provides the shape of the braking component and points of attachment of the braking component to a greater mechanism, such as a brake, transmission or car. The metal substrate, when attached to a greater mechanism, transfers the frictional force generated by the friction material to the greater mechanism, for example, to stop a car.
The metal substrate may be any known or conventional metal used in the manufacture of brakes, clutches or structural metal components. Examples of metals include ferrous metals (for example, steels and cast iron), aluminum, aluminum alloys, titanium, titanium alloys, magnesium and magnesium alloys. Preferably the metal of the metal substrate is a ferrous metal, aluminum or aluminum alloy. More preferably the metal is aluminum or alloy thereof.
The friction material is laminated to at least a portion of a face of the metal substrate such that, under normal operating conditions, the friction material is the only part of the braking component that contacts an opposing component to provide the frictional force. For example, when the braking component is a brake rotor, the friction material is laminated to the braking face of a metal rotor (that is, metal substrate) where the braking face of the rotor is the area contacted and swept by a brake pad upon braking. The friction material may be laminated to the metal substrate in segments or continuously. That is to say, there may be gaps between the CMC laminated to the metal substrate as long as the friction material is the only part that contacts, under normal operation, an opposing component to generate the frictional force. An illustrative example is a brake rotor that has pads of friction material that are uniformly distributed around and laminated on the braking face of the metal brake rotor. Generally, the friction material covers from 10 percent to 100 percent of any particular face.
The frictional material may be any thickness depending on, for example, the particular braking component (for example, truck brake versus car brake), desired lifetime of the component and severity of the environment the braking component may operate in. Generally, the thickness of the braking component is from 0.5 to 20 mm. Preferably, the thickness is from 1 to 10 mm.
The Ceramic-Metal Composite (CMC)
The friction material is a ceramic-metal composite (CMC) that is comprised of a ceramic phase and a metal phase dispersed within each other. Herein, the CMC is understood to contain essentially no resinous binder (for example, phenol-formaldehyde resins), except that which may penetrate open pores of the CMC when it is glued to the metal substrate using an adhesive described under xe2x80x9cPreparing the Braking Component.xe2x80x9d Otherwise, essentially no resinous binder is an amount corresponding to at most a trace amount in the body of the CMC.
The metal phase of the CMC may be a metal selected from the Periodic Table Groups 2, 4-11, 13 and 14 and alloys thereof. Said groups conform to the new IUPAC notation, as described on pages 1-10 of the CRC Handbook of Chemistry and Physics 71st Ed., 1990-91. Preferred metals include silicon, magnesium, aluminum, titanium, vanadium, chromium, iron, copper, nickel, cobalt, tantalum, tungsten, molybdenum, zirconium, niobium or mixtures and alloys thereof. More preferred metals are aluminum, silicon, titanium and magnesium or mixtures and alloys thereof. Most preferably the metal is aluminum and alloys of aluminum, such as those that contain one or more of Cu, Mg, Si, Mn, Cr and Zn. Exemplary aluminum alloys include Alxe2x80x94Cu, Alxe2x80x94Mg, Alxe2x80x94Si, Alxe2x80x94Mnxe2x80x94Mg and Alxe2x80x94Cuxe2x80x94Mgxe2x80x94Crxe2x80x94Zn. Specific examples of aluminum alloys include 6061 alloy, 7075 alloy and 1350 alloy, each available from the Aluminum Company of America, Pittsburgh, Pa.
The ceramic phase of the CMC may be a boride, oxide, carbide, nitride, silicide or combination thereof. Combinations include, for example, borocarbides, oxynitrides, oxycarbides and carbonitrides. Generally, at least about 45 volume percent of the ceramic phase has a melting or decomposition temperature of at least about 1400xc2x0 C. Preferably at least about 60 percent, more preferably at least about 80 percent and most preferably at least about 90 percent by volume of the ceramic phase has a melting or decomposition temperature of at least about 1400xc2x0 C. Preferred ceramics include SiC, B4C, Si3N4, Al2O3, TiB2, SiB6, SiB4, AlN, ZrC, ZrB, a reaction product of at least two of said ceramics or a reaction product of at least one of said ceramics and the metal of the CMC. The most preferred ceramic is boron carbide.
Examples of a ceramic-metal composite include B4C/Al, SiC/Al, AlN/Al, TiB2/Al, Al2O3/Al, SiBx/Al, Si3N4/Al, SiC/Mg, SiC/Ti, SiC/Mgxe2x80x94Al, SiBx/Ti, B4C/Ni, B4C/Ti, B4C/Cu, Al2O3/Mg, Al2O3/Ti, TiN/Al, TiC/Al, ZrB2/Al, ZrC/Al, AlB12/Al, AlB2/Al, AlB24C4/Al, AlB12/Ti, AlB24C4/Ti, TiN/Ti, TiC/Ti, ZrO2/Ti, TiB2/B4C/Al, SiC/TiB2/Al, TiC/Mo/Co, ZrC/ZrC/ZrB2/Zr, TiB2/Ni, TiB2/Cu, TiC/Mo/Ni, SiC/Mo, TiB2/TiC/Al, TiB2/TiC/Ti, WC/Co and WC/Co/Ni. The subscript xe2x80x9cxxe2x80x9d represents varying silicon boride phases that can be formed within the part. More preferred combinations of a metal and ceramic include: B4C/Al, SiC/Al, SiB6/Al, TiB2/Al and SiC/Mg. Most preferably, the CMC is comprised of a chemically reactive system, such as aluminum-boron carbide or aluminum alloy-boron carbide. In a chemically reactive system, the metal component can react with the ceramic during formation of the CMC resulting in a new ceramic phase being formed. Said new phase can modify properties, such as hardness and high temperature strength of the composite. A most preferred chemically reactive system is B4C/Al, wherein the metal phase is aluminum or alloy thereof and the ceramic phase is comprised of at least two ceramics selected from the group consisting of B4C, AlB2, Al4BC, Al3B48C2, AlB12 and AlB24C4.
To impart, for example, sufficient wear resistance, the ceramic phase of the CMC is at least about 20 percent by volume of the CMC. However, the amount of ceramic phase in the CMC should not be so great that, for example, it is difficult to bond the CMC to the metal substrate adequately. The ceramic phase is preferably present in an amount of at least about 50 percent, more preferably at least about 75 percent and most preferably at least about 85 percent by volume to preferably at most about 98 percent by volume of the CMC.
In a preferred embodiment of the CMC, the metal phase is non-contiguously dispersed within the ceramic phase and, consequently, the ceramic phase is interconnected. In this preferred embodiment, the metal phase is comprised of regions that preferably have an average equivalent diameter of at most about 30 microns, more preferably at most about 10 microns, and most preferably at most about 5 microns, and preferably at least about 0.25 micron, more preferably at least about 0.5 micron, and most preferably at least about 1 micron. Preferably the largest metal region is at most about 100 microns, more preferably at most 75 microns, and most preferably at most 50 microns in diameter. In addition, it is also preferred that the metal regions are predominantly equiaxed and predominantly situated at ceramic-ceramic grain triple points as opposed to elongated along ceramic grain boundaries, as determined by optical quantitative stereology from a polished sample described by K. J. Kurzydtowski and B. Ralph, The Quantitative Description of the Microstructure of Materials, CRC Press, Boca Raton, 1995.
The CMC may be porous as long as the CMC provides sufficient wear resistance, heat dissipation and strength during operation, for example, of a brake or a clutch. The porosity may advantageously vary cross the thickness of the CMC laminated to the metal substrate. For example, the face of the CMC laminated to the metal substrate may be more porous than the face that contacts an opposing component. The porosity may enhance the heat shielding of the metal substrate and also, may aid the bonding of the CMC to the substrate. Generally, the porosity, given by the percent of theoretical density of the CMC, is preferably at least about 90 percent, more preferably at least about 95 percent and most preferably at least about 98 percent of theoretical. Herein, the theoretical density is the theoretical density described on page 530 of Introduction to Ceramics 2nd Ed., W. D. Kingery et al., John Wiley and Sons, New York, 1976.
The CMC may have any density, so long as the CMC provides sufficient wear resistance, heat dissipation and strength under operating conditions, for example, of a clutch or brake. Since it is advantageous for a brake inter alia to be as light as possible, the CMC preferably has a density of at most about 6 g/cc, more preferably at most about 4 g/cc, and even more preferably at most about 3 g/cc to preferably at least about 0.5 g/cc, more preferably at least about 1.0 g/cc and most preferably at least about 1.5 g/cc.
The dynamic coefficient of friction of one component""s CMC in contact with another component""s CMC should provide a frictional force sufficient to operate, for example, a brake or clutch under operating conditions, but not so high that excessive wear or heat is generated. More specifically, the dynamic coefficient of friction of the CMC, in contact with another CMC in motion relative to each other, is desirably at least about 0.2. The dynamic coefficient of friction may be determined by a pin on disk method using a 1 pound load, as described by ASTM G-99 Standard and M. A. Moore, in xe2x80x9cWear of Materials,xe2x80x9d pp. 673-687, Am. Soc. Eng., 1987. The CMC coefficient of friction is preferably at least about 0.3, even more preferably at least about 0.4, more preferably at least about 0.6 and most preferably at least about 0.8 to preferably at most about 5.
The wear resistance of the CMC against itself is desirably an amount sufficient to provide a greater lifetime (that is, greater wear resistance) than the CMC against cast iron. For example, it is preferred that the CMC has a wear diameter of less than about 5 mm, more preferably less than about 1.5 mm, and most preferably less than about 1 mm, as determined by the pin on disk method described in the previous paragraph.
The toughness of the CMC may be any toughness sufficient to avoid catastrophic failure of the CMC under operating conditions, for example, of a brake or clutch. Preferably the toughness is at least about 5.0 MPamxc2xd. More preferably the CMC toughness is at least 5.5, even more preferably at least 6 and most preferably at least about 6.5 MPamxc2xd to preferably at most about 25 MPamxc2xd, as determined by a Chevron Notch method described in xe2x80x9cChevron-Notched Specimens: Testing and Stress Analysis,xe2x80x9d STP 855, pp. 177-192, Ed. J. H. Underwood et al., Amer. Soc. for Testing and Matl., Pa., 1984.
The thermal conductivity of the CMC should be great enough to dissipate the heat generated during operation, such that the CMC (particularly the CMC surface in contact with an opposing CMC surface) is not damaged by excessive heat. To dissipate the heat generated during operation (for example, braking), the CMC advantageously has a thermal conductivity of at least about 5 W/m-K, as determined by a laser flash method described in more detail by xe2x80x9cFlash Method of Determining Thermal Diffusivity, Heat Capacity, and Thermal Conductivity,xe2x80x9d Journal of Applied Physics, W. J. Parker et al., 32, [9], pp. 1679-1684. More preferably the thermal conductivity is at least about 10, even more preferably at least about 20 and most preferably at least about 25 W/m-K. However, the thermal conductivity should not be so great that the metal substrate is damaged due to excessive heat. For example, it is desirable that the CMC has a thermal conductivity less than about 150 W/m-K.
The CMC should also have a specific heat such that the temperature reached during contact with an opposing component is less than a temperature sufficient to damage either the CMC, metal substrate or other metal components that make up a larger mechanism, such as a brake or clutch. Preferably the specific heat is at least about 0.4 J/gxc2x0 C., at room temperature as determined by differential scanning calorimetry. More preferably the specific heat is at least about 0.6, even more preferably at least about 0.8 J/gxc2x0 C., and most preferably at least about 1 J/gxc2x0 C. to preferably at most the maximum theoretically possible for a selected CMC. The specific heat also desirably increases as the temperature increases. For example, the specific heat at 1000xc2x0 C. is desirably at least double the specific heat at room temperature.
The flexure strength of the CMC may be any strength sufficient to avoid fracture of the CMC under operating conditions. For example, the strength of the CMC, at about room temperature, should be at least about 150 MPa as determined by ASTM C1161. Preferably the strength is at least about 200 MPa, more preferably at least about 300 MPa, and most preferably at least about 400 MPa to preferably at most about 1500 MPa. It is more preferred that the CMC have the above strengths at about 500xc2x0 C., even more preferably at about 700xc2x0 C. and most preferably about 900xc2x0 C.